Observation of Dark Spots and Dark Lines of GaN Microcrystals Grown by Nitridation of Gallium Sulfide

High-spatial-resolution (50 nm) cathodoluminescence images in the ultra-violet region (365 nm) and in the yellow region (570 nm) were taken at room temperature for GaN bulk crystals under a scanning electron microscope equipped with a monochromator and a real-time RGB-separation detector. The crystals were grown by nitridation of Ga 2 S 3 and had a c-facet and side facets with Miller indices of [110x] or [112y]. Dark spots were observed only on the side facets while dark lines only on a c-facet. Dark lines stretch in the direction of (1100) or (11200) and end as a dark spot on side facets. Dark spot density was 10 9 cm -2 . The nonuniformity of yellow luminescence and its irrelevance to the existence of dark lines suggest an extrinsic defect for the origin of yellow luminescence, such as carbon impurity.